2006 GMC Sierra Transmission Problems

The 2006 GMC Sierra was the last full model year of the GMT800 platform.  There was a limited run of “classic” Sierras that ran in with the GMT900 in 2007.  These trucks are over ten years old.  It is hitting the point that your 2006 Sierra may be having transmission problems.  Below, you’ll find a ton of troubleshooting your transmission problems.  The 2006 Sierra was a carryover from the 2005 model year.

It is necessary to determine what kind of transmission is in the truck.  The easiest and quickest way to go about determining this is by RPO codes.  The RPO codes are a list of production options that are printed and stuck in your truck;s inner glove compartment when it is manufactured.  You’ll find everything you’d want to know the options on your truck from this list.  RPOcodes.com is a great resource to lookup all the codes for your Sierra beyond the transmission codes that are listed above.

Troubleshoot 2006 GMC Sierra Transmission Problems
Use the RPO codes in your glove compartment to determine which transmission you need to troubleshoot.

The transmission options that were available to the 2006 Sierra were as follows:

Common 2006 Sierra Transmission Problems

This section is being presented to you as a top-level troubleshooting guide, to help you determine what’s wrong with your 2006 Sierra’s transmission, and what is needed to fix it.  Most of the advice and information is applicable to the two automatic transmissions that came in almost all these trucks.  The NV3500 is a lot simpler piece of machinery.

Or, you can go to any of the troubleshooting guides for these transmissions:

4L80E Transmission Problems–  This transmission came in some of the heavier duty HD truck models.

4L60E Transmission Problems–  The 4L60E was far and away the most common transmission that came in these trucks.

Check Engine Light–   Before doing any serious work, if you have a check engine soon light, go ahead and have it scanned to see what trouble codes it is throwing out.  You might find the problem right there.  AutoZone, or any other parts store will be able to help you with this for free.  It really is worth noting that there doesn’t need to be a trouble code thrown for there to be a code in the computer.

What to do When Your Transmission Light Comes On


No 1st and No Overdrive–  When the truck has no overdrive and no first gear it is almost certainly in limp mode.  Limp mode is designed to keep the transmission from hurting itself when it detects something is wrong.

A couple of things to know about limp mode are that it is not going to ruin your transmission to run it in limp mode, but it isn’t good for it either.  That’s because when entering limp mode the line pressure in the transmission is increased to its maximum pressure.  Another thing is, if you keep it in drive you’re only getting 3rd gear, but if you manually shift it into 2nd gear, it’ll go there.  That makes taking off much better on the transmission.  In either first or second gear, the torque converter slips more than it normally would on a takeoff.

Most of the time, limp mode is caused by loss of signal to/from the TCM.  Either the wiring harness has a short or the transmission fuse is blown.  It doesn’t matter whether your 2006 Sierra has a 4L60E or 4L80E, the fuse will be in the same location. We recommend starting there..  You might get lucky.

Here’s exactly how to find the 4L60E/4L80E fuse location.


No Reverse–  Most of the time reverse won’t engage when the clutches are worn out.  It may be (but is less likely) that the sun shell went bad as well.  There’s a whole article on this site dedicated to diagnosing no reverse in the 4L60E.  It is pretty applicable to the 4L80E as well.

Transmission Not Locking in Overdrive–  The 4L80E and 4L60E both have torque converters that lock.  That is to say when they are moving at a certain speed, they’ll lock and create a direct 1:1 connection between the engine and transmission.  It removes all of the torque converters multiplication factor and increases drivetrain efficiency.

When the torque converter doesn’t lock, it can feel weird or jarring.  You’ll go down the highway and the engine rpm will be higher and it feels almost as if the transmission is over reacting to what you are telling the gas pedal.

This is usually caused by the lockup circuit failing.   There’s a switch under the brake pedal that works like a brake light switch that tells this circuit when it is time to activate.  Both sides will be hot when it is working properly.

See also:  4L60E No Overdrive Diagnosis

Not Moving in any Gear–  If your 2006 Sierra Transmission has a problem moving in any gear, it is likely caused by a lack of transmission fluid.  No matter how silly that sounds, and how convinced you are that it is not the problem, go ahead and check it anyway.  You should want this to be the problem.  It is way less expensive than replacing a bad torque converter.

Low Transmission Fluid Signs

Automatic Transmission Wont Go in Gear- Troubleshooting

Good luck with your Sierra!